The present invention is directed to an ignition device for igniting a heating boiler having a combustion portion operating according to the fluidized bed principle.
Small heating boilers using solid fuels such as coal, peat and wood, are usually based upon grating combustion. There has been extensive experience with boilers having grating. Such boilers are simple and inexpensive to construct. However, drawbacks of such boilers include their low efficiency, and combustion products therefrom which are harmful to the environment, especially when the boiler is operating with a small load.
Automatic fuel feed into such a boiler is expensive, when the fuel is of large size (e.g. cordwood or chopped fire wood). With many kinds of fuel such as peat and straw pellets or certain grades of coal, fusing of ash and sintering thereof into a cake upon the grating impedes the use of such fuels. The sintered slag is removable with the aid of various mechanical mechanisms, however in many instances, such mechanisms interfere with the combustion in a small boiler.
Small boilers with grating combustion of the prior art have numerous disadvantages. The efficiency is poor with varying loads. Suitability of such small boilers for varying types of fuel is also unsatisfactory. Moreover, operation of such small boilers is labor-intensive.
Fluidized bed boilers are large in size and complex in construction. Therefore, these type of boilers which are known at present are not applicable to the class of small boilers.
A fluidized bed boiler is ignited either electrically, or, in the case of larger boilers, with oil or gas. When oil or gas is used, ignition takes place in a manner such that an oil or gas burner directly heats the bed of sand or the hot flue gases used for fluidizing. In the latter situation, a fluidized air blower blows air into a separate pre-heating chamber, which is provided with an oil or gas burner. The burner draws its air for combustion from the fluidized air flow.
A small fluidized bed boiler has been electrically ignited, in that an electric heating resistance has been placed in the input line leading to the air supply chamber of the boiler, as disclosed for example in U.S Pat. No. 4,183,308. In that case, a normal electrical air heater is involved. The drawback of this particular prior art device, is the poor heat transfer to the air.
In Finnish Patent Application No. 82 3267, an electrical ignition is disclosed, wherein the electric heating element has been disposed in the combustion portion, operating according to the fluidized bed principle, of the boiler. In this prior art device, the heat transfer from the resistance to the fluidized sand is efficient, but not uniform, as a consequence of which the resistance may be locally overheated. Since the resistance has been disposed in the lower portion of the sand layer, which is at rest during combustion, this device requires ample space, with the quantity of sand that must be heated at ignition being large. It is furthermore difficult to automate the ignition, because the ignition has two phases and requires, for instance, two sets of air nozzles.